I have been replacing a fuse box with a consumer unit and doing a load of small electrical jobs (saved up for years) on a house in a local village. Supply is overhead, 3 phases come in; one connected. House is TT although many houses in the village are PME. The final job was sort out why no power in the outbuildings.

Outbuildings are on their own mcb in main CU in house. There is a MEM CU with 3 circuits in the outbuildings with a 30mA rcd as the main switch, which was permanently tripped. Outbuildings on their own TT. Disconnected all the neutrals and switched off the 3 x mcbs and the rcd still tripped. It had a functional earth connected and disconnecting that stopped it tripping. Connecting back up the neutrals with functional earth disconnected and it all works fine. Rcd operates within spec for x½, x1 and x5. Ramp test has it tripping at 27mA.

So I have left the functional earth disconnected as I believe it is providing as much protection as a 'standard' rcd. I have not used rcds with functional earths before, but understand it might be detecting a voltage on the neutral. I could measure 8Vac on the neutral with respect to local outbuilding TT earth. I was not aware of any local high current loads. House owner said she first noticed problem after a big thunderstorm 5 weeks ago, but that might be a coincidence rather than a cause.

Has anyone had this before? RCD looks in good condition and apart from tripping when functional earth is connected it appears to work fine. Might there be a DNO issue in the local network. Any thoughts gratefully received.

Well, I can do sums. Supply voltage at the time today was 230 ish. I had measured it at 240 the previous day. No significant load on in the house. No obvious cannabis farm in neighbours house, nor agric or industrial loads nearby. Electricity pole is in street outside the house and at first sight has an earth. So I was surprised at the 8v I measured on the neutral and wondered whether that should be enough to make the functional earth function and trip the rcd.

I might not have been clear. The 0.6 ohms LN loop was in the outbuildings on the end of a bit of 2.5mm2 cable run overhead fused at 20A at the main board. The same test at the origin gave 0.35 ohms. So I do not get the relevance of your 12kVA point.

Hi bajb, my understanding is that without the functional earth connected the device will not operate in the event of a neutral to earth fault, rather than the above mentioned loss of neutral protection scenario. (if the supply was TN-C-S, loss of neutral could also result in the loss of earth, so the functional connection to earth wouldn't be much help! )

As you say the device connected normally still tripped without any neutrals connected on the outgoing side etc, I would probably say the device is faulty and in need of replacement.

WB, thanks for you comments. House is TT, outbuildings are separate TT. I have been back today and double checked my results. The rcd, with functional earth disconnected, passed all the standard tests of x1/2, x1 and x5 and gives a 27mA ramp test result on both 0 and 180 degrees. It also trips with a neutral to earth fault.

So my conclusion is that the sensing circuit related to the functional earth was probably damaged in the thunderstorm which I am now told took out a neighbours computer. What it will not do I believe is detect or operate correctly in a lost neutral scenario (the manufacturer's data for the recent version talks of the latter).

The customer does not want to do anything about it as it is still providing the protection a new, less sophisticated rcd would provide. I should change it but I am not doing it for free.

It does seem a bit curious why MEM decided to have a functional earth connection on these RCD's (I have come across these before but don't know of any other make of RCD that has functional earth other than those found on RCBO's) - the chances of needing an RCD to operate for it's intended purpose when there happens to be a loss of supply neutral must be pretty slim! ??

I guess the crux of the matter would be would you be prepared to put your decision to leave the functional earth disconnected down in writing? If you contact MEM and they agree that without the functional earth connection the device will operate to provide the protection as the per less sophisticated devices required by the regs then it will be OK, but for anyone following up at a later date without any documentation, the missing connection could be a cause for concern?

Don't get me wrong on this, not having a jab, you've done all the tests etc, questions are purely theoretical for discussional purposes open to all

A dangerous condition can arise if the neutral wire is broken or switched off on the supply side of the RCD, while the corresponding live wire remains uninterrupted. If the tripping circuit needs power to work, it cannot operate. Connected equipment will not work without a neutral, but the RCD cannot protect people from contact with the energized wire. For this reason circuit breakers must be installed in a way that ensures that the neutral wire cannot be switched off unless the live wire is also switched off at the same time. Where there is a requirement for switching off the neutral wire, two-pole breakers (or four-pole for 3-phase) must be used. To provide some protection with an interrupted neutral, some RCDs and RCBOs are equipped with an auxiliary connection wire that must be connected to the earth busbar of the distribution board. This either enables the device to detect the missing neutral of the supply, causing the device to trip, or provides an alternative supply path for the tripping circuitry, enabling it to continue to function normally in the absence of the supply neutral.

Without disecting that wikipedia entry, please do bear in mind that persons such as a certain Mr c*ckburn may have had a hand in its content!.
...............................................................................................................
Like yourself, do not intend to, never was sore on Cockburn he had his ideas: However another point, we all know that the test button on a double pole RCD is across live and neutral via a resistor so designed to give a current two and half times the tripping current when the button is pressed. This is why it is better to press the button before RCD instrument test to awake the RCD up out of maybe its 5-year slumber. I put it to you (because I don't know) is the test button circuit across live and neutral or live and earth in a single pole RCBO (the ones have the earth tail that is). Because if you lost the neutral I suppose the button would still work if it was connected live to earth TT, TN-S your TN-C- S lost neutral there is always water pipes, human body etc